Taming Lasers to Create Pure Single Photons for Quantum Tech

Quantum photonic technologies promise to revolutionise computing, but they hinge on a difficult prerequisite: generating sources of single photons that are perfectly identical. Nanowire-based quantum dots have long been viewed as ideal candidates for this task. However, achieving 'coherent control'—precisely driving these dots using resonant lasers—has proven notoriously difficult. The primary obstacle is the laser itself; the intense light needed to excite the dot creates overwhelming backscatter, drowning out the delicate single photons researchers aim to capture.

In a significant breakthrough, researchers have established a reliable technique to solve this via a tapered single-mode nanowire waveguide. By complementing standard polarisation rejection with a method called 'mode-matching', they effectively minimised the troublesome backscattered laser light. This alignment allows the excitation laser to couple efficiently with the quantum dot while preventing noise from reflecting back into the detection channel.

The results confirm the efficacy of this approach. The team demonstrated low multiphoton emission and observed multiple 'Rabi oscillations'—a rhythmic fluctuation between quantum states that signals successful coherent control. Furthermore, they reported a two-photon indistinguishability visibility of 41 per cent under resonant excitation. This represents a substantial improvement over incoherent excitation methods. While perfection remains on the horizon, this scalable approach marks a vital step toward the high-quality, coherent single-photon sources required for advanced quantum interference.